1. Field of the Invention
The present invention relates generally to adaptive equalizers, which may be used to compensate for signal transmission by way of a channel having unknown and/or time-varying characteristics such as may occur in high definition television reception and, more particularly, relates to an equalizer/forward error correction (FEC) automatic mode selector.
2. Background of the Invention
In the Advanced Television Systems Committee (ATSC) standard for High Definition Television (HDTV) in the United States, the equalizer is an adaptive filter which receives a data stream transmitted by vestigial sideband modulation (VSB), VSB being the modulation system in accordance with the ATSC-HDTV standard, at an average rate equal to the symbol rate of approximately 10.76 MHz. The equalizer attempts to remove or reduce linear distortions mainly caused by multipath propagation, which are a typical characteristic of the terrestrial broadcast channel. See United States Advanced Television Systems Committee, “ATSC Digital Television Standard,” Sep. 16, 1995.
Decision Feedback Equalizers (DFE's) as used in the communications art generally include a feedforward filter (FFF) and a feedback filter (FBF), wherein typically the FBF is driven by decisions on the output of the signal detector, and the filter coefficients can be adjusted to adapt to the desired characteristics to reduce the undesired distortion effects. Adaptation may typically take place by transmission of a “training sequence” during a synchronization interval in the signal or it may be by a “blind algorithm” using property restoral techniques of the transmitted signal. Typically, the equalizer has a certain number of taps in each of its filters, depending on such factors as the multipath delay spread to be equalized, and where the tap spacings “T” are generally, but not always, at the symbol rate. An important parameter of such filters is the convergence rate, which may be defined as the number of iterations required for convergence to an optimum setting of the equalizer. For a more detailed analysis and discussion of such equalizers, algorithms used, and their application to communications work, reference is made to the technical literature and to text-books such as, for example, “Digital Communications”, by John G. Proakis, 2nd edition, McGraw-Hill, New York, 1989; “Wireless Communications” by Theodore S. Rappaport, Prentice Hall PTR, Saddle River, N.J., 1996; and “Principles of Data Transmission” by A. P. Clark, 2nd edition, John Wiley & Sons, New York, 1983.